The manufacturers of machine tools that remove material from work pieces by means of electrical discharge machining are generally confronted with the problem that despite all their efforts when cabling the working area, it is very difficult to transmit short, rectangular processing pulses to the spark gap.
This is particularly the case for current amplitudes that are greater than approx. 5 A and pulse durations that are shorter than approx. 0.5 μs. The cause of a pulse deforming in a broadening and flattening manner is the parasitic leakage inductance between the pulse source and the spark discharge, which even in the best case could exceed 300 nH.
One option would be to increase the pulse source voltage but this is not possible because it must be possible to adjust the pulse voltage independently as it is one of the most important process parameters.
The physics involved in electrical discharge machining is particularly interesting in the range of pulse durations that are shorter than approx. 100 ns because below this limit the machining process changes from an in-part melting off process to a purely evaporating process, so-called ablation. The temperature gradient at the discharge site changes so abrupt that the work piece material does not experience any structural changes.
The condensate of this metal vapour is, insofar as it is not dissolved in the dielectric as metal oxide or metal hydroxide, only present in the dielectric as extremely fine grain particles. This leads to a further important advantage, namely to a suppression of the uncontrollable and damaging lateral discharges, across larger granules of eroded material, by way of example when precision drilling.
A similar development is recognized when considering the recent history of laser cutting that is closely related to electrical discharge machining. Partly unexpected good machining results relating to removal rate and surface quality were achieved using ever shorter beam pulses, down to femto seconds, and also higher pulse powers. Improvements occurs quantum related when discrete limit values for the pulse duration and beam power were exceeded.
With regard to the electrical discharge process that as far as the physical principle is concerned functions in a similar manner, the hitherto chosen path has been in the direction of ultra-short pulses for the above mentioned reasons. It follows from this that fundamentally new solutions are required in order to effectively reduce the parasitic leakage inductance at the discharge site.
Coaxial cables or also strip conductors have been used for almost 100 years as pulse-shaping devices. The main application fields have been in high energy physics for the nuclear research industry, ultrasound generators, and radar systems and in cell biology research.
JPS56-119316A from Mitsubishi discloses feed lines having in each case a capacitor. However, the rise times of a feed line having a capacitor are far too long. Rectangular pulses are likewise equally difficult to achieve.
FIG. 4 of the present application illustrates a known solution that uses coaxial cables. The patent EP 1 719 570 from the inventor D'Amario, wherein already existing power cables are used in an electrical discharge wire-cutting machine in an innovative manner as a pulse-forming device in order to achieve a specific pulse power of 100 kW/mm2 Pulse currents of 36 A after 150 ns pulse duration or 43 A after 190 ns pulse duration are quoted and were found to be sufficient for use in the middle machining range prior to the polishing range.
D'Amario includes reference to the patent DE 26 53 857, Ullmann et al and in doing so refers to the fact that it is possible to further improve the current rise time by using sliding contacts.
It has likewise been known for a long time to use capacitor elements and inductance elements as delay elements arranged in rows as substitute homogenous delay lines, the so-called discrete delay lines.
Discrete delay lines were also proposed in the U.S. Pat. No. 3,033,971 from Pfau by way of example as early as 1957 for an electrical discharge generator that does not comprise any electronic switching means. The solution illustrated in FIG. 5 demonstrates a decoupling impedance R, L for charging the discrete delay line, and a saturable reactor Ls as a switching element. The voltage-time integral of the saturable reactor Ls can be varied by way of pre-magnetization. A resistor that is parallel to the spark gap ensures that saturation can occur in a stable manner in the event that the spark gap is in a very high ohmic state.
The poor scalability of all solutions that are based on coaxial cables or strip conductors must definitely be mentioned as a disadvantage of the prior art. If the object is to achieve namely higher pulse power-s-, then this solution is immediately extremely voluminous and cost-intensive.
The patent EP 1 719 570 from D'Amario makes an exception in this case because coincidentally the cables that are suitable for the object of his invention having a material value of approx. Euro 1,000.—were already available on the machine.
However, D'Amario also does not conceal in his patent EP 1 719 570 that theoretically he had expected a pulse of 50 A at 42 ns (accordingly to 1.2 A/ns) but had measured only a pulse of 36 A at 150 ns, (accordingly 0.24 A/ns). This was obviously absolutely sufficient to achieve the object of his invention, however, a consequent further development of electrical discharge technology demands more radical approaches.
The main reason why many solutions that use coaxial cable produce such unexpected poor results for the pulse shape resides in the inability to achieve an almost inductance-free connection between the bulky cables and the load. The lower the charging voltage is, the greater will be the inductance problem. It is therefore not surprising that a large portion of relevant publications refer only to theoretical simulations.
The main disadvantage of the generator in accordance with U.S. Pat. No. 3,033,971 from Pfau resides in the additional large residual inductance of the saturable reactor Ls in the saturated state that is also added to the leakage inductance of the cabling. As a result, this solution is completely unsuitable for generating extremely short pulses. Further disadvantages relate naturally also to the missing protection against short circuits and arcing on the spark gap, and also that the process parameters cannot be controlled independently.